Effective integrated weed management starts early and varies by species

Weeds contribute to crop yield loss every year. In addition to weed abundance challenges, there is a growing list of herbicide-resistant weeds in Western Canada, both in terms of the herbicide groups that they are resistant to and the locations that weed species have been confirmed in.

Prairie weed overview
The new Prairie Weed Monitoring Network (PWMN) showcases provincial weed survey results on weed abundance and herbicide resistance. It also features maps of various herbicide-resistant weeds in Saskatchewan and an informative Herbicide Resistant Weeds in the Prairie Region poster (pictured below) of Prairie weed species that have resistance to select herbicide groups (based on their mode of action). The prevalence and distribution of herbicide-resistant weeds depicted in these resources emphasizes the need for additional weed management strategies to supplement chemical weed control options.

As explained in this paper, integrated weed management (IWM) includes a combination of cultural, physical, or biological control strategies in addition to herbicide-based management options. No single strategy is expected to manage all weeds in all crops with IWM. Instead, it includes multiple options that can together offer a more robust longer term solution if effectively implement. Since every generation of the weed life cycle has the potential to produce exponentially more weeds, early adoption of integrated weed management is the best chance for success.

Herbicide resistance research
Tidemann’s five-year (2016-2020) Mitigating herbicide resistance – investigating novel integrated weed management systems project, which builds on previous research, examined IWM tactics to manage wild oat, wild buckwheat, and locally-important broadleaf weeds (ex. cleavers at Lacombe and kochia at Lethbridge) at six Prairie locations (Beaverlodge, Lacombe, and Lethbridge, AB; Scott and Saskatoon, SK; and Carman, MB).

Weed populations were measured for plots with diverse crop rotations (canola-wheat, faba beans-barley-canola, peas-winter wheat-canola, silage barley-winter triticale-silage barley, silage barley-fall rye-canola, alfalfa-alfalfa-alfalfa), with and without herbicides (various combinations), typical or increased seeding rates, and with and without chaff collection.

To allow weed populations to establish, trials were seeded to wheat at double the seeding rate and not treated with herbicides in year one, and again in year five. The diverse crop rotations were planted over years two, three and four.

Both the impact of a single species as well as a full weed community (such as a broadleaf species, a grass weed species and other dominant weed species in the area) were investigated, along with economic considerations. Managing multiple weed species (and each of their life cycles) is challenging in research and on farms, especially when strategies that control one species may not address another species.

Weed community and species-specific findings

Wild buckwheat populations were not significantly affected (on average) by any of the crop rotations or integrated weed management techniques tested, across locations. This may have been due to the twining/climbing nature of buckwheat which makes it more resilient to the IWM strategies that rely on increased crop competitiveness to reduce weed impacts.

As noted in a previous blog, the grass weeds – especially wild oats – were found to be more competitive than broadleaf weeds (and can reduce the size and impact of broadleaf weeds). For instance, the reduction in broadleaf weed biomass in the low diversity, no herbicide treatment may have been due to the high and competitive wild oat populations in the same plots.

Wild oat management strategies that were previously studied (ex. increased seeding rates and inclusion of a winter cereal or barley silage) weren’t always effective for controlling wild oats or other species in this study. The research team suggested that this could be the result of IWM strategies being less effective on high weed populations. This emphasizes the need for early adoption, because once resistance is established (and densities increase) it will be much harder to control.

The use of silage barley and winter cereals provided some efficacy in managing weed communities or species. But the economics and marketability of these crops would need to be considered before it can be considered a recommended solution.